Communications : Extended Protocol

Introduction

This document will describe the Extended Communications Protocol as implemented with InfoSight Marking System software. Extended Protocol is intended to provide secure communications with an intelligent host device. This document assumes familiarity with the ASCII character code as well as fundamental computer programming skills.

Disclaimer

This document is correct to the best knowledge of InfoSight Corporation which reserves the right to change the document's contents or the systems referred to at any time without prior notification.

Protocol Parameters

Communications is established via an RS232 serial connection with the following parameters:

Software handshaking (XON/XOFF) is implemented in some types of marking systems to control the flow of information. Hardware handshaking (DTR/DSR) is not implemented.

Communications with the marking system controller is accomplished via a Master / Slave arrangement with the Host being the Master and the marker being the slave. The slave will only transmit in response to a message from the master.

Note: All transmissions are in standard ASCII utilizing the following control characters:

Master Data Format

            SOH    TYPE    STX    [DATA TEXT]    ETX    [BCC]    CR

Where,

TYPE - A single printable ASCII character that defines the meaning and the contents of the message [DATA TEXT] field. Message types may be custom defined for certain applications as required. Standard message types are defined later.

[DATA TEXT] - An optional field which contains the actual data of the transmission. Some message types require no data since the "message" is conveyed by the TYPE character.

BCC - This is an optional field used to improve link reliability by providing fault detection. The BCC is computed by taking an eight bit addition of the TYPE and DATA TEXT characters and transmitting them as a three digit ASCII decimal number in the range 000 to 255. Refer to the example BCC computation later in this document.

Slave Data Format (response)

The slave will respond to the master's transmission in one of two ways depending on whether errors were detected or not.

         SOH    TYPE    ACK    STX    [DATA TEXT]    ETX    BCC    CR
or
         SOH    TYPE    NAK    STX    [DATA TEXT]    ETX    BCC    CR

If no errors were detected in the reception of the packet, then the first response will be sent back to the master. If any errors were detected (e.g. Parity, Framing, Overrun, BCC, Format, etc.) then the second (NAK) message will be sent. Note that the ACK message does not necessarily imply that the DATA TEXT field itself is correctly presented, just that no communications errors occurred.

The TYPE character will always be the same as the received TYPE.

The DATA TEXT field is optional and depends on the message TYPE. The BCC field will always be present in the response.

Retries

If the host does not receive a response from the I-Dent within three seconds, or it receives a NAK response, it should retransmit the entire packet. If, after three retries (four tries total), the host has not received a response, the host should declare the link to be "down".

Standard Message Types

Certain message types which are common to most marking systems are briefly discussed below. For a complete description of the Extended Protocol message types for a particular marker, refer to the data sheet for that marker.

1 - The DATA TEXT is a message to be printed. The message text is stored in the buffer for the currently assigned buffer. The response message contains no data in the DATA TEXT field.

A - The DATA TEXT is a number from 1 to 10 specifying the buffer number to Assign for printing. The response message DATA TEXT field will contain an ASCII '1' if the assignment number is valid, or '0' if it is invalid.

S - This type is used to query the marker for its current status. There is no DATA TEXT field for this type. The ACK response message from the marker will contain a DATA TEXT field. This field will contain one or more numbers separated by commas where each number is made up of four ASCII digits representing a decimal value. The binary equivalent value of this number has a bit pattern representing the status of the marker and is dependant upon the specific marker and application.

Example BCC Computation

The following example is a typical transmission including the BCC.

To download the character string 'ABC123' to the currently assigned message buffer, send the following message.

         SOH    1    STX    ABC123    ETX    141    CR

where '1' is the message TYPE and 141 is the BCC. The BCC is computed as follows (note all math shown in hexadecimal):

1) BCC = Message TYPE character + DATA TEXT characters.

		031H		1	- Message Type
		041H		A	\
		042H		B	|
		043H		C	| message Text
		031H		1	|
		032H		2	|
	+	033H		3	/
	______________
		18DH

2) We are only interested in the lower eight bits of the sum, so we discard the first digit and keep the lower two. This results in a BCC of 8DH. Note that when performing the summation in an eight bit variable (e.g. unsigned char in 'C') that the most significant bits are automatically truncated. If the master's programming language is incapable of doing eight bit addition, then the same result can be obtained by taking the MODULO 256 operation on a sixteen bit sum. The MODULO operation is division where the Remainder is kept and the Quotient is discarded.

3) Once the BCC value is obtained, it must be placed into the message packet after the ETX character. The BCC must be transmitted in its decimal ASCII form. The decimal equivalent of the hexadecimal value 8DH is 141 decimal. Converting the value 141 into three ASCII characters yields 031H, 034H and 031H. These three characters become the transmitted BCC.

The actual data transmitted (in hex) by the host for this message is:

         001H    031H    002H    041H    042H    043H    031H
         032H    033H    003H    031H    034H    031H    00DH

4) If the marker receives the message correctly, it will respond with the following message:

         001H    031H    006H    002H    003H    030H    034H    039H    00DH

which equates to the following ASCII message:

         SOH    1    ACK    STX    ETX    049    CR

Note that if the marker detected an error in receiving the message from the host it would respond with a NAK (015H) character in place of the ACK. The BCC would be unaffected by this since the ACK/NAK is not included in the BCC computation.

Conclusion

This concludes the description of the Extended Protocol for communications with InfoSight Corporation Marking System Software. Please refer all questions to the factory.